<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    #wrapper {
        width: 960px;
        margin: 0 auto;
    }
    #mainContent {
        border-right: 220px solid #dfdfdf;
        position: absolute;
        width: 740px;
        height: 800px;  
        background: green;
    }
    #sidebar {
        background: #dfdfdf;
        margin-left: 740px;
        position: absolute;
        height: 800px;
        width: 220px;
    }
</style>
<body>
    <div id="wrapper">
        <div id="mainContent">...</div>
        <div id="sidebar">...</div>
    </div>
</body>
</html>